1. Field of the Invention
The present invention relates to a circuit for prevention of unintentional writing to a memory. More specifically, it relates to a circuit equipped with a low-voltage detection circuit capable of preventing unintentional writing to a nonvolatile memory when the low-voltage detection circuit detects a drop in power supply voltage. The present invention also relates to a semiconductor device equipped with such circuit.
2. Description of the Background
The following description sets forth the inventor's knowledge and should not be construed as an admission that the description constitutes prior art.
A nonvolatile memory, such as an EPROM, an EEPROM and a flash memory capable of erasing stored data in blocks instead of one byte at a time, can retain stored data even when the power supply voltage is turned off. Consequently, nonvolatile memory commonly is used in devices such as microcomputers to store programs such as BIOS, so that the programs can be easily updated to fix problems or bugs discovered during evaluation and use.
However, even though it is able to retain stored data when power is turned off, a nonvolatile memory becomes unstable in operation when power is first turned on or when the power supply VDD experiences transient drops in voltage. This is because a nonvolatile memory such as a flash memory has charges flowing into a floating gate, and sufficient charges cannot flow into the floating gate in the state of a low power supply voltage. As a result, the prescribed data holding characteristics cannot be assured. To overcome such a problem, a low-voltage detection circuit conventionally is provided, which directly resets the state of the nonvolatile memory itself.
The low-voltage detection circuit detects a drop in power supply voltage (such as when power is first turned on, or as a consequence of “noise” in the power supply), and automatically sends a reset signal to a writing/reading control portion of the nonvolatile memory. Thus, in a situation where the power supply voltage VDD is low, writing of data to the memory is prohibited. As a result, writing to the nonvolatile memory will be performed only when it is appropriate to execute the writing operation.
In recent years, as mentioned above, nonvolatile memories have been used in many devices, including microcomputers. The microcomputer and the nonvolatile memory conventionally are integrally formed as a single chip on a semiconductor substrate. A low-voltage detection circuit is connected to the power supply circuit of the integrated microcomputer and nonvolatile memory to detect a low-voltage status of the power supply voltage VDD.
In this semiconductor device, in cases where it is configured that the entire system is reset when the low-voltage detection circuit detects a power supply voltage (VDD) drop, there is a drawback that it takes a significant amount of time to change the system back to the default setting.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. Indeed, certain features of the invention may be capable of overcoming certain disadvantages, while still retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.